1. Field of the Invention
This invention relates generally to transmission and switching techniques in telephone communication systems and, more particularly, to an improved conference technique whereby a number of channels in a telephone switching system employing pulse code modulation (PCM) for transmission purposes are combined so that a number of subscribers may participate in a common telephone conversation. It also relates to improvements in a three-port conference circuit of the type disclosed in U.S. Pat. Nos. 3,699,264 and 4,007,338 both of which are assigned to the same assignee as the present invention.
2. Background Art
An approach to handling of pulse code modulated information and conference circuitry is taught by U.S. Pat. Nos. 3,699,264 and 4,007,338. In these patents, digital signals are not converted to analog; rather binary words are compared from the participating channels, with the smallest binary numbers (this corresponds to the largest analog signal) selected as the speaker with all other parties of the conference designated listeners. The speaker's speech sample is distributed as a listening sample to all the conference listeners. The conference circuit, in U.S. Pat. No. 4,007,338, determines the loudest PCM voice sample (smallest binary number) during each PCM time frame, storing and outputting the selected PCM code to all conferees. This technique, called "instant speaker selection", for generating conferencing however, is subject to degradation due to the presence of idle channel noise and DC offset voltage variations from non-talking conferees talking over the speaker when the speaker's audio signal passes through a null and its PCM sample is represented by a high numeric value. In addition, when two or more conference members are conversing simultaneously, the conference circuit could alternately and sporadically select a new speaker during each successive time frame, thus degrading the quality of the speech heard by the conversing conferees.
Various improvements in the conference circuitry disclosed in the above-identified U.S. patents are disclosed in U.S. Pat. Nos. 4,002,981 and 4,054,755, both of which are also assigned to the same assignee as the present invention.
In U.S. Pat. No. 4,022,981, an improved multi-port (more than 3) conference circuit is disclosed utilizing a minimum binary code as employed in the coding formats (D2 and D3) currently employed in pulse code modulated telephony. Generally, the method of choosing the speaker here is to clear the PCM buffers at the end of a time slot frame. then, when the first conferee is detected, its speech sample is loaded into a conferee register. The conferee register is then compared to a temporary speaker register and if of larger amplitude (smaller binary PCM code value), the conferee register sample is loaded into the temporary speaker register. If the sample in the conferee register is not of larger amplitude register, it is written over when the next conferee sample is loaded into the conferee register. Each new conferee sample is thus compared to the temporary speaker register to determine which conferee has the largest amplitude sample for a given frame. Finally, at the end of a given frame, the temporary speaker register is transferred to a conference speaker register and becomes the conference speaker sample which all conferees except the speaker himself will receive during the next frame.
In U.S. Pat. No. 4,054,755, further improvements in a multi-port conference circuit are provided. These improvements attempt to solve the idle channel noise, the circuit offset variation problem and also provide foreign signal protection (i.e., 60 Hz signal longitudinally coupled to the line).
In this conference circuit, PCM samples are taken for each conferee from the time switch and, via comparator circuits, a PCM sample is selected for sending to the conferee. The selected PCM sample is not determined until all samples within a frame are compared which results in all conferees except the selected conferee receiving the selected PCM sample during the next succeeding frame. The conferee whose sample was selected will receive a null code (perfect idle channel). To minimize speech clipping or selecting noise, two circuits, a preliminary and a preferred speaker preference circuit, are employed.
The preliminary preference circuit utilizes the identity of the previous selected speaker and after its PCM sample is compared, its binary weight is modified to the highest value of a corresponding curve segment. This is done by adding a bit between the segment and the step bits, causing the binary value of the step to be decreased thereby increasing the sample's amplitude. This technique permits the conference circuit to hold on to the previous speaker if the incoming PCM samples are below or in the same PCM segment or in value.
The preferred speaker preference circuit functions to prefer a selected speaker when the magnitude of the selected speaker PCM sample exceeds a preferred speaker preference circuit threshold. In this regard, when a speaker sample is selected during a given frame for use as the selected speaker during the succeeding frame and the sample has a larger PAM (smaller PCM code) sample than the threshold, the preferred preference circuit adjusts the selected speaker sample to a lower binary weight (appaently larger PAM sample value) when sending it to the comparator, during the succeeding frame. This reduces speech clipping during that time when two or more conferees are conversing simultaneously by giving preference to the conferee presently designated speaker of the conference.
Neither the preliminary nor the preferred preference circuits alters the incoming or the outgoing PCM sample to the conferees; the additional binary weights are only presented to the comparison circuit for the purpose of favoring the previous speaker.
U.S. Pat. No. 4,126,766 teaches improvements in the conference circuit taught in the above-referenced U.S. Patents to reduce or substantially eliminate the problem of high idle channel noise resulting from always choosing the largest signal above null code (quiet or absence of signal).
This invention provides a three-port conference circuit which includes a plurality of time slots, during which each of two port samples may be compared with a strapped threshold and with each other to determine which is to be selected as the speaker for the purpose of transmission of that port's sample to a third port.
If the sample of one of the two compared ports exceeds the threshold and simultaneously is greater than the other compared port's sample when the other port is designated the speaker from the last frame in a last frame speaker memory, that port will be designated the speaker for this frame, its sample will be transmitted to the third port and it will be designated the speaker in the last frame speaker memory. For all other conditions, the port designated the speaker in the last frame speaker memory will remain the speaker and its sample will be transmitted to the third port. This speaker selection will take place separately for each of the parties of a three-way conference during that party's time slot.
A method of conferencing, described in the GTE Automatic Electric Technical Journal (Vol. 13, No. 2, page 71) is taught wherein eight-bit PCM code samples are converted to 13-bit linear code, the conference samples added, and the sum reconverted back to eight-bit PCM code. Such a method suffers from distortion caused by loss of sample accuracy when, in the process of reconverting the sum back to an eight-bit PCM sample, surplus bits are discarded.
U.S. Pat. No. 4,109,111 issued Aug. 22, 1978 to Cook teaches a pulse code modulation, time division multiplex conferencing system providing to each party a signal indicative of the linear sum of the samples from all other parties, such signals generated by accumulating the sum of all samples from all parties then generating an output signal to a particular party by subtracting from the sum the contribution of the particular party. The cumulative sum is updated upon receipt of each new sample.
Such an arrangement while operating generally satisfactorily requires the conversion of each sample from PCM format to a linear format in order to perform the arithmetic operations described. Additionally, time consuming and complex circuitry is required for such conversion.